Fix IWYU in fxcrt headers.

core/include/fxcrt/fx_string.h

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef _FX_STRING_H_
 #define _FX_STRING_H_
+
+#include "fx_memory.h"
+
 class CFX_ByteStringC;
 class CFX_ByteString;
 class CFX_WideStringC;
 class CFX_WideString;
 struct CFX_CharMap;
 class CFX_BinaryBuf;
 typedef int FX_STRSIZE;
 class CFX_ByteStringL;
 class CFX_WideStringL;
 
@@ skipping 845 matching lines @@
 CFX_ByteString  FX_UTF8Encode(FX_LPCWSTR pwsStr, FX_STRSIZE len);
 inline CFX_ByteString   FX_UTF8Encode(FX_WSTR wsStr)
 {
     return FX_UTF8Encode(wsStr.GetPtr(), wsStr.GetLength());
 }
 inline CFX_ByteString   FX_UTF8Encode(const CFX_WideString &wsStr)
 {
     return FX_UTF8Encode(wsStr.c_str(), wsStr.GetLength());
 }
 #endif